The invention relates to a porous electrode for a battery, and more peculiarly, to a binder and/or electrolyte material for tie preparation of a porous electrode in a battery and to a porous electrode comprising such a binder and/or electrolyte material. The invention also relates to a method for the production of such a binder and/or electrolyte material.
It is of great importance for electrodes that they have a high surface area resting in lower current densities, and consequently lower overvoltages for the electrode reactions. It has been found that this is obtained by employing porous electrodes.
Porous electrodes are currently used in many commercial batteries of today. This type of electrode brings several advantages, one of them being a large specific surface area. Another advantage is that the electrolyte is partly present in the porous structure, and hence higher limiting currents can be reached in short times, as compared to when a planar electrode is used. This is due to the outer limiting mass transport for the latter type of electrode.
The compactness of the porous electrode has a great impact on the ohmic potential drop because it determinates the distance through which the current flows. The latter effect is especially important when the battery is discharged/charged at high rates. High rates often lead to severe transport limitations in the depth of the electrode, and consequently to a low utilisation of the active electrode material. To minimise the latter internal loss of electrode performance, it is of great importance that the conductivity in all of the components of the electrode is high, and that the internal contact between the electrode components is good.
In principal, the electrode preparation may consist of mixing the electrochemically active materials together with carbon black and a binder dissolved in an organic solvent, e.g. cyclo-hexane, to a homogenous slurry. The slurry is thereafter evenly spread on a current collector, for instance by means of a Doctor blade, or pressed directly on the current collector, e.g. an aluminium foil. Finally, the electrode is left to dry. An intercalation compound such as LiMn2O4, LiCoO2 or LiNiO2 is normally provided on the electrode if used as a cathode, and a carbonaceous material such as coke or graphite if used as an anode.
JP-A-53010032 describes an electrode containing a core material of a conductive porous material, e.g. a nickel screen and an active material layer e.g. of hydroxide powder, carbonyl nickel powder as a conductive material and a binder, e.g. of CMC ethyl cellulose or PVA, PVC. The electrode is intended to be used in an alkaline storage cell, to obtain low polarisation and long life time.
One problem is that most common binders consist of an olefinic thermoplastic rubber, such as EPDM or the like. However, polyolefins are very poor ion conductors, which is a serious disadvantage for this type of product. The low ion conductivity results in high ohmic drop over the depth of the electrode. Some parts of the electrode may be totally inaccessible for ions such as Li-ions due to blocking effects caused by the binder. These two factors may severely limit the overall performance of the battery cell. Therefore, carbon black is added to the binder in order to compensate for the low conductivity.
Another problem is that poor contact between different materials often occurs.
It should, therefore, be appreciated that there is a need for a binder and/or electrolyte material, an electrode and related method for producing the material, to solve these problems. The present invention fulfills this need.
The invention is embodied in a binder and/or electrolyte material being intended to be used in the preparation of a porous electrode for a battery cell, a porous electrode comprising such a material, and related method, for producing the martial, said binder and/or electrolyte material providing high ion conductivity and adhesiveness by incorporating ion conducting polymer segments and adhesive polymer segments into the binder martial of polymeric type.
This leads to higher ion conductivity and improved adhesive properties in a single binder and/or electrolyte material.
According to a preferred embodiment of the invention, the ion conducting segments comprise at least one poly(alkylene oxide), preferably poly(ethylene oxide).
The binder and/or electrolyte material according to one preferred embodiment of the invention comprises a modified copolymer or block copolymer having ion conducting segments and adhesive segments incorporated.
Advantageously, the ion conducting segments a linked to at least one epoxidisable polymer chain segment. If the copolymer is a block copolymer, the ion conducting segments are preferably lined to a poly-isoprene or polybutadiene chain of the block copolymer.
The electrode according to one embodiment of the invention comprises an electrochemically active material, such as an intercalation compound, or an insertion carbonaceous material, a binder and/or electrolyte of polymeric type, advantageously provided on a current collector, wherein the binder and/or electrolyte material comprises a polymer having ion conducting polymer segments incorporated, preferably a copolymer, wherein the ion conducting polymer segments are incorporated with adhesive polymer segments.
The invention is also embodied in a method for the production of a binder and/or electrolyte of polymeric material intended to be used in the preparation of a porous electrode for a battery, wherein a copolymer is modified by incorporating ion conducing polymer segments and adhesive polymer segments.
According to a preferred embodiment of the invention a suitable block copolymer of xe2x80x9cstar typexe2x80x9d block having xe2x80x9cchainsxe2x80x9d containing carbon-carbon double bonds, for example polyisopreneyl is employed, which are epoxidised and thereafter hydroxylated to obtain polymer segments carrying hydroxyl groups, which may be utilised as initiators for ethoxylation by at least one poly(akylene oxide) to obtain poly(alkylene oxide), preferably poly(ethylene oxide) chains, which are ion conductive.
According to another preferred embodiment of the invention, a suitable copolymer is EPDM.
High ion conductivity is combined with good adhesive properties in a single binder material, which will increase the cell performance as compared to conventional cells using EPDM/carbon black.